Develop the photoresist until all unexposed areas become wet and detach.

Rinse PCB under warm tap water.

Try #1

Ambient temperature: 26°C.

This try was intentionally made with minimal precautions, to establish a quality
baseline and determine which of the preparation steps are actually essential.

PCB was run through laminator once.

Photoresist was developed for 4 min.

Photoresist adhesion was poor. I’ve accidentally brushed against a corner before exposing
and photoresist has detached from copper together with top protective film.

Quality: very low; very rough edges.

Photoresist was removed and board was reused for next try.

Try #2

Ambient temperature: 24°C.

PCB was run through laminator five times.

Photoresist was developed for 4 min.

Quality: better, varying. Edges are nicer, but thinnest line was not present.
NaOH application from try #1 might have had an effect.

Photoresist was removed and board was reused for next try.

Try #3

Ambient temperature: 26°C.

This try intentionally follows try #1 to determine whether cleaning with NaOH
had any positive effect.

PCB was run through laminator once.

Photoresist was developed for 4 min.

Quality: better edges than try #1, worse adhesion than try #2.

Photoresist was removed and board was reused for next try.

Try #4

Ambient temperature: 26°C.

PCB was run through laminator five times.

Photoresist was developed for 4 min.

After developing, PCB was gently scrubbed with a soft acrylic brush under warm tap water.

Quality: moderate. Visible gaps can be attributed to dust between photoresist and PCB.
Smallest features are missing, but in uniform way; 0.15mm (6 mil) line is reliably reproduced.

Photoresist was removed and board was reused for next try.

Try #5

Ambient temperature: 26°C.

PCB was run through laminator five times. Photoresist was not entirely attached
to PCB before lamination; only the front edge was pressed upon PCB.

I’ve forgot to measure the time photoresist was developed for, but it was likely
more than 4 minutes.

While developing, PCB was gently scrubbed with a soft acrylic brush after resist started
to visibly weaken / detach from board.

Quality: poor. It appears I have overdeveloped the resist. However, while I was
brushing it in the solution, even smallest elements were clearly visible, much
better than when I scrubbed it under tap water. It is likely that agitating the board
reduces the time and solution concentration needed for developing.

Ideas to try:

Begin to scrub the board immediately and not when resist starts to visibly detach.

Reduce concentration of developer solution.

Photoresist was removed and board was reused for next try.

Try #6

Ambient temperature: 26°C.

This try verifies idea #1 from try #5.

PCB was run through laminator five times. Photoresist was not entirely attached
to PCB before lamination; only the front edge was pressed upon PCB.

While developing, PCB was gently scrubbed with a soft acrylic brush immediately
after immersion.

Quality: very good. For the first time, 0.1 mm line is not damaged. However, it is likely true
that developer needs to have lower concentration.

Photoresist was removed and board was reused for next try.

Try #7

Ambient temperature: 26°C.

This try verifies idea #2 from try #5.

After cleaning, PCB was thoroughly wiped with lint-free cloth.

PCB was run through laminator five times. Photoresist was not entirely attached
to PCB before lamination; only the front edge was pressed upon PCB.

New developer solution was used with concentration 2%wt.

While developing for 1 min, PCB was gently scrubbed with a soft acrylic brush
immediately after immersion.

Quality: best so far. It appears that being thoroughly wiped of all residues of isopropanol
is one of key requirements for good photoresist adhesion. Additionally, 2% developer
solution didn’t work slower (or worse) than 5% one—if something, it was faster—an unexpected
result. The board is a bit underdeveloped, however.

Photoresist was removed and board was reused for next try.

Try #8

Ambient temperature: 26°C.

After cleaning, PCB was thoroughly wiped with lint-free cloth.

PCB was run through laminator five times. Photoresist was not entirely attached
to PCB before lamination; only the front edge was pressed upon PCB.

Developer solution used had concentration 2%wt.

While developing for 2 min, PCB was gently scrubbed with a soft acrylic brush
immediately after immersion.

Quality: nearly perfect.

Interlude: water break test

I’ve noticed that the very first try, despite horribly selected conditions, yielded one
of the best imprints. I’ve also noticed that each time I clean the PCB from exposed
(and unexposed) resist, there remains a visible ghost picture of the traces from previous
try. Most of PCB (unexposed areas) is also clearly hydrophobic—meaning unexposed resist
is not cleaned well enough.

I’ve decided to figure out the best way to clean board, using water break test to verify
its condition. (Water break test in a nutshell: really clean copper is hydrophilic,
contaminants are hydrophobic. Place PCB under water. If it’s really clean, a smooth,
unbroken layer of water will remain.)

Test #1

PCB is immersed in NaOH for 2-4 min to remove old exposed resist, then scrubbed with
isopropanol several times. (The procedure I’ve used earlier.)

Result: entire PCB is clearly hydrophobic. (Huh?!)

Test #2

PCB is immersed in NaOH for 2-4 min to remove old exposed resist, then scrubbed with
isopropanol several times, then immersed in NaOH for 10 min again.

Result: entire PCB seems to be hydrophilic. I don’t have a standard to compare with,
so I’m assuming this is “good enough”.

Try #9

Ambient temperature: 26°C.

PCB was cleaned with isopropanol, then immersed in NaOH for 10 min.
Afterwards, PCB was rinsed in tap water and thorougly wiped with lint-free cloth.

PCB was run through laminator five times. Photoresist was not entirely attached
to PCB before lamination; only the front edge was pressed upon PCB.

Developer solution used had concentration 2%wt.

While developing for 2 min, PCB was gently scrubbed with a soft acrylic brush
immediately after immersion.

Quality: perfect. No defects due to poor photoresist adhesion are present; all visible
defects are also present on photonegative. All features down to 50 µm are reproduced.
(Photoresist resolution is about 40 µm.)